1. Field of the Invention
The present invention relates to an apparatus and method for controlling colors of a color image, and more particularly, to an apparatus and method for controlling saturation according to the brightness or hue of each pixel and also controlling hue according to the brightness of the pixel.
2. Description of Related Art
In general, when the same color of a color image is represented by diverse image displays, output colors are different from one another due to a difference in spectral characteristics of the displays. And, even when the same color is represented by identical image displays, output colors may be different. Since the same color should be represented completely equally according to brightness or hue by the diverse displays as well as the identical displays, delicate control of color needs to be done for color matching. Such color control eliminates a color difference made between the diverse displays or between the identical displays, and with currently increasing interests in colors, meets users' demands for selective color control so that they can convert a specific color represented by a display into a preferable color.
Conventional methods of controlling colors of a color image in a color imaging system are disclosed in U.S. Pat. No. 4,525,736, U.S. Pat. No. 6,057,931, U.S. Pat. No. 6,122,012, and U.S. Pat. No. 6,476,877. U.S. Pat. No. 4,525, 736 discloses a selective color modification, which converts red (R), green (G), and blue (B) signals of a color picture into luminance (Y), in-phase(I), and quadrature (Q) signals, selects a color gamut, within which color modification is to take place, in a YIQ three-dimensional color space of the converted signals or in a two dimensional image, and adds modification amounts MY, MI, and MQ to the signals only when the three color signals are found to lie simultaneously within the selected color gamut. However, in the method a color to be controlled is difficult to select and there is a possibility of discontinuity between a selected gamut and a non-selected gamut in the color space.
U.S. Pat. No. 6,057,931 discloses a method and apparatus for controlling color image reproduction in a color printer. The method adjusts a hue-saturation-brightness (HSL) slice range, within which a color to be manually or automatically controlled falls, in an HSL color space, and controls respective color components within a set HSL slice using a transfer curve. Here, at most seven HSL slices can be set to control the color. However, since the respective color components are not independent in the HSL color space but instead correlate with one another to some degree, it is difficult to control only one desired color component. Particularly, color control cannot be achieved according to brightness or hue because of the transfer curve.
U.S. Pat. No. 6,122,012 discloses a method of selective color control of digital video images, in which the saturation and brightness of a specific color are controlled by defining a set of look up tables, each of which is a function showing saturation components of six colors, namely, red (R), green (G), blue (B), cyan (C), magenta (M), and yellow (Y), and dynamic ranges of the saturation components, and by performing a linear interpolation in the look up tables. However, this method has a drawback in that a display system should have a memory with a capacity large enough to store the huge look up tables and the look up tables should be kept for future reference. Thus, the method has a limitation in controlling colors. In addition, although the method can control colors according to hue, it cannot control the colors according to brightness.
U.S. Pat. No. 6,476,877 discloses a color correction apparatus, a color correction controller, and a color correction system. Color correction is performed by converting a color space using given RGB color signals into a color space in which H, S, and L are represented, and by performing correction between two colors obtained from different displays in the converted color space. Here, the color space is divided into six gamuts and it is determined whether the two colors lie within the same gamut. If the two colors are determined to lie within the same gamut, color correction is performed in accordance with a correction procedure for the two colors within the gamut. If the two colors are determined to lie in different gamuts, the different gamuts are combined to a single gamut so that the two colors can lie within the single gamut. Thereafter, the above correction process is repeated. In order to match the two colors having different hue and saturation in the singe gamut, other colors, marked by spots, in the gamut are also adjusted in proportion to variations in the hue and saturation components of the two colors. However, in this method, since the color gamut to be controlled has already been determined, uniformity of the color space may be severely degraded during color control. Furthermore, since the method does not consider brightness, the colors cannot be controlled according to brightness.
FIG. 1 illustrates an original YCbCr color space in which all colors, which can be combined in an RGB color space, are distributed. In the YCbCr color space, Y represents a brightness component of a color, and Cb and Cr represent saturation components. Although the brightness component and the saturation components are separately shown in the YCbCr color space, the saturation components are not constant according to brightness and hue. Accordingly, when the respective color components are controlled in accordance with a control function, the controlled components may exceed a range that allows the color to be represented. In other words, the respective color components in the YCbCr color space are not independent of each other and have a certain relation with one another. Thus, if the color is controlled without considering the relation, the color may not be represented.
FIG. 2 is a diagram illustrating an exemplary color control in the original YCbCr color space. If saturation components of colors C and A, which are marked by spots, equally increase without considering brightness, the color A is moved to a spot B and the color C is moved to a spot D. Since the modified darker color D lies within a color gamut, it can be represented by a display. However, the modified brighter color B lies outside the color gamut, and thus cannot be represented by the display. To represent the color B by means of the display, an additional process, such as clipping, should be conducted. Further, since brightness components and saturation components, which are color components in the YCbCr color space, affect each other, their relation needs to be considered during color control.
Conversely, human beings can perceive the difference between the brightness components and the saturation components. Therefore, it is advantageous in many ways that when colors of an image are processed, a color signal is represented with the brightness components and saturation components. In a hue-saturation-value (HSV) color space or a hue-saturation-intensity (HSI) color space based on this concept, however, the respective color components are not independent. Accordingly, it is not easy to control the color components.